A method for treating a purge stream derived from a sodium carbonate, sesquicarbonate, wegsheiderite, or bicarbonate crystallizer,said purge stream comprising sodium carbonate and/or sodium bicarbonate and at least 1% by weight of sodium chloride and/or sodium sulfate,the method comprising: causticizing at least 50 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate into a caustic solution and into a calcium carbonate mud with lime and water, separating the calcium carbonate mud from the caustic solution; concentrating the caustic solution by removing part of the water to obtain a concentrated caustic solution comprising at least 25% NaOH, and a crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate, separating the crystallized solid from the concentrated caustic solution, said crystallized solid to be disposed of or to be further valorized, recycling part of the concentrated caustic solution to the sodium carbonate, sesquicarbonate, wegsheiderite, or bicarbonate crystallizer.